1. Field of the Invention
The present invention relates to a hydraulic pressure control device for an automatic transmission, and more specifically, to a hydraulic pressure control device for an automatic transmission provided with a plurality of sets of a friction engaging element and an electromagnetic valve to supply or discharge hydraulic oil to the friction engaging element.
2. Background Art
In recent years, realization of a smooth and highly-responsive gear shift feeling has been sought by a so-called clutch-to-clutch control method in which engagement and disengagement of clutches are performed simultaneously by directly supplying and cutting off oil from a hydraulic source by electromagnetic valves.
Hydraulic pressure control devices of automatic transmissions that realizes the above method have basically a hydraulic circuit configuration in which one electromagnetic valve is disposed in each of friction engaging elements and forms each shift stage by allowing two of the friction engaging elements to engage each other, except in the case where one-way clutch (O. W. C) for a first shift stage is used or where an electromagnetic magnetic valve is shared.
In the above configuration, it is necessary to assume that a so-called interlock occurs in which three or more of the friction engaging elements might engage one another due to a hardware failure or a software failure of the electromagnetic valves. Accordingly, in JP No. 2689421, four normally high type electromagnetic valves (whose output is ON at the time of disconnection or non-excitation and that is hereinafter also referred to as “NH”) are used for four friction engaging elements, and fail-safe valves are provided between the friction engaging elements and the electromagnetic valves. If interlock occurs, communication between predetermined friction engaging elements and electromagnetic valves is cut off and the friction engaging elements are brought into communication with a drain, which results in disengagement of the friction engaging elements.
However, in the configuration of JP No. 2689421, all the electromagnetic valves are of normally high (NH) type. Therefore, if disconnection occurs, all the electromagnetic valves have an output in an ON state similar to an ON failure and the fail-safe valves operate. Thus, if sticking occurs at the fail-safe valves, it will result in interlocking. Accordingly, in JP-A No. 10-122349, a configuration is provided in which NH type electromagnetic valves are used for two friction engaging elements that form a given shift stage at the time of disconnection, normally low type electromagnetic valves (whose output is OFF at the time of disconnection or non-excitation and that is hereinafter also referred to as “NL”) are used for the other friction engaging elements, and fail-safe valves are disposed between the NL electromagnetic valves and the friction engaging elements. According to this configuration, if disconnection occurs, a shift stage at the time of the disconnection is attained without operation of the fail-safe valves. In addition, if the NL type electromagnetic valves have failed in an ON state, the fail-safe valves operate so that the interlock can be prevented, similar to JP No. 2689421.
In JP No. 2689421 and JP-A No. 10-122349 described above, a configuration is disclosed in which oil is drained by disposing the fail-safe valves on the downstream side (on the side of the friction engaging elements) of the electromagnetic valves. However, in JP-A No. 2000-35110, the fail-safe valves are disposed on the upstream side of the electromagnetic valves, and if the fail-safe valves operate, disengagement of the friction engaging elements is performed by cutting off the supply pressure of the electromagnetic valves.
Further, in the configuration of JP-A No. 2001-248724, O. W. C. for a 1st shift stage is provided and four friction engaging elements and the O. W. C. for the 1st shift stage compose forward shift stages, but the fail-safe valves are disposed on the upstream side similar to JP-A No. 2001-35110. Further, in a configuration in which electromagnetic valves and control valves are combined together, even when the electromagnetic valves are normal, the control valves may stick (sticking of a valve member), which makes the output OFF. However, in JP-A No. 2001-248724, by making a configuration in which electromagnetic valves, control valves and changeover valves are combined together, the output can be made ON even when the control valves stick solely or the changeover valves stick solely. As a result, a vehicle can travel without the shift stage being changed to a neutral position (N) at the time of a failure.